Optical characterization entails illuminating a sample with electromagnetic radiation and receiving radiation emitted from the sample responsive to its illumination. It is often desirable to increase the signal received from the sample given a fixed level of illumination (i.e., increase efficiency) and/or to increase the spatial resolution of the measurement, and various methods have been developed to accomplish these purposes. In particular, it is often desired to provide high spatial resolution and high efficiency simultaneously. Due to the diffraction limit, special methods are required to provide high efficiency at sub-wavelength spatial resolution. For example, sub-wavelength resolution can be provided by placing a sub-wavelength aperture between the radiation source and the sample, and placing the sample in the near field of the resulting aperture radiation pattern. However, radiation transmission efficiency through a sub-wavelength aperture tends to be very low, so the resulting scheme provides enhanced spatial resolution but significantly reduces efficiency.
Various approaches have been considered for addressing this problem. US 2005/0084912 considers a nanolens including plasmon resonance particles used to emit radiation for near-field sample characterization. Resonant enhancement is exploited to increase efficiency. U.S. Pat. No. 6,781,690 considers microcavities in combination with fractal nanoparticles, where efficiency is increased by cavity resonance and resonance within aggregates of fractal nano-particles. In US 2005/0218744, a medium having randomly distributed metallic particles near the percolation threshold is considered for optical characterization. In US 2005/0031278, a C-shaped sub-wavelength aperture is considered for increasing efficiency while maintaining high spatial resolution.
Although the approaches considered above should provide improved performance compared to a simple circular (or square) sub-wavelength aperture, there is room for further improvement in efficiency and spatial resolution, since no optimal aperture shape appears to be known. Accordingly, it would be an advance in the art to provide such improved combinations of efficiency and spatial resolution.